Pi-conjugated heavy-metal polymers particularly suited to hydroponic applications

ABSTRACT

A class of organic light-emitting diodes (OLEDs) emit light tailored to the absorption spectra of growing plants. Upon photoexcitation, the OLEDs generate light in both the blue-green and red regions of the visible spectrum. A heavy metal atom present in the π-conjugated polymer chain (such as platinum and/or iridium) acts via the spin-orbit coupling mechanism to cause an enhancement of the ratio of fluorescent to phosphorescent emission to be of approximately equal strength. These two emission bands overlap the absorption spectra of common plants grown under hydroponic conditions.

REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Patent Application Ser. Nos. 60/799,893 and 60/799,891, both filed May 12, 2006, the entire content of both of which are incorporated herein by reference.

GOVERNMENT SPONSORSHIP

Research carried out in connection with this invention was supported in part by National Science Foundation, Grant No. DMR-05-03172; and United States Department of Energy, Grant No. FG-04ER46109. Accordingly, the United States government may have certain rights in the invention.

FIELD OF THE INVENTION

This invention relates generally to organic light-emitting devices and, in particular, to π-conjugated heavy-metal polymers particularly suited to plant growth and hydroponic applications.

BACKGROUND OF THE INVENTION

Organic light-emitting materials have great commercial potential in a number of areas, including light-emitting devices and displays. Organic materials offer potential advantages in low-cost fabrication, large-area and mechanically flexible devices, and the availability of diverse molecular-structure property relationships.

A conventional polymer electro-luminescent device comprises a thin film of electro-luminescent polymer sandwiched between two electrodes. Polymer electro-luminescent devices are described, for example, in U.S. Pat. Nos. 5,247,190 and 5,399,502 to Friend et al., in U.S. Pat. No. 4,356,429 to Tang, 4,672,265 to Eguchi et al., 4,539,507 to VanSlyke and Tang. The entire contents of these patents are incorporated herein by reference.

Efficiency is an important parameter in device design, and is related to the ratio of light energy out to electrical energy in. The quantum efficiency of a device is related to the number of photons emitted relative to the number of charge carriers introduced to the emissive material. Quantum efficiency is in turn dependent on a number of factors, which are discussed in more detail below.

If device efficiency can be increased, brighter displays are possible for the same electrical input. Alternatively, electrical input can be reduced for the same light output, which saves energy and may increase the lifetime of the display, another important design parameter.

Conjugated polymers are often used in organic electro-luminescent devices. These polymers typically comprise a backbone having alternating single and double carbon-carbon bonds, such that extensive electron delocalization occurs. When polymers of this type are used as conducting polymers, an oxidizing agent may be added to remove an electron from a polymer double bond. The remaining lone electron, associated with a positive charge due to the removal of an electron, can then propagate along the polymer chain under the influence of an electric field. This propagating charge is known as a polaron. Reducing agents may be used to donate additional electrons to the chain, which may also propagate along the chains as polarons.

In electroactive devices using conjugated polymers, a polymer film is typically in contact with two electrodes. Electrons are injected into the polymer at one electrode, and electrons are withdrawn from the polymer at the other electrode. The withdrawal of electrons is usually termed hole injection, as the absence of the electron, or hole, propagates in the manner of a positively charged charge carrier. The injected electrons propagate as negative polarons, the injected holes propagate as positive polarons. Electro-luminescence may occur due to the interaction of positive and negative polarons, as discussed below. This interaction may sometimes be termed recombination or annihilation of carriers.

Within the organic layer, charge-transfer (CT) reactions occur between a positively charged polaron (P⁺) and a negatively charged polaron (P⁻). The polarons are associated with two participating locations (such as polymer chain segments), and each polaron has spin ½. The interaction between the two oppositely charged polarons leads to the formation of an intermediate encounter complex, involving both locations, and then to the formation of a final state. The final state comprises the ground state of one participant and an excited state of the other participant. The excited state may be either a neutral exciton singlet state (S) or a neutral exciton triplet state (T). Light emission occurs only for singlet exciton decay.

The current state of the art devices for visible lighting are incandescent light bulbs (7 to 22 lumens per watt—efficiency about 2.5%), fluorescent lamps (efficiency about 11%) and LED devices. Each of these figures refers to the entire visible light spectrum. Plants absorb only in the blue-UV and deep red regions of the spectrum, so any light in me green-yellow-orange and even red-orange portion of the spectrum is wasted.

SUMMARY OF THE INVENTION

A class of polymers has been synthesized and characterized which, upon photoexcitation, emits light in both the blue-green and red regions of the visible spectrum. A heavy metal atom present in the polymer chain (such as platinum and/or iridium) acts via the spin-orbit coupling mechanism to cause an enhancement of the ratio of fluorescent to phosphorescent emission to be of approximately equal strength. These two emission bands overlap the absorption spectra of common plants grown under hydroponic conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart showing experimental data;

FIG. 2 shows the absorption of Pt-3 (solution) along with excitation at 362 nm;

FIG. 3 is a plot of a photoluminescent measurement of a polymer (Pt-3) solution;

FIG. 4 is a plot of photoluminescent measurement of Pt-3 polymer film; and

FIG. 5 is a plot of photoluminescent measurement of Pt-3 polymer at different temperatures and phase variation.

DETAILED DESCRIPTION OF THE INVENTION

Organic light-emitting polymers are well known in the art to comprise polymers having a high degree of π conjugation along their backbones. These polymers are electrically conducting, and as further known in the art, the light emission which they are capable of producing may be controlled and tailored by controlling the degree of conjugation of the π electrons, as well as by controlling the nature and number of organic side groups on the polymer backbone. As described hereinabove with reference to the present invention, further enhancement and control of the light emission may be had by incorporating metals into the light-emitting polymers. These metals are, in particular instances, heavy metals; and, noble metals comprise one particular group of heavy metals which may be utilized in the present invention.

In view of this teaching, one of skill in the art could readily synthesize the metal-containing, π-conjugated, light-emitting polymers of the present invention without undue experimentation. Methods and techniques for the preparation of such polymers are well documented. In previous work, we described suitable mixtures, showing how a heavy atom may be used to tune emission spectra to include the generation of white and near-white light. One particular synthesis of a metal-containing, π-conjugated polymer having utility in such organic light-emitting devices will be described.

In the first step of the preparation of the material, a mixture of diethynlbenzene, bromophenylethynl-TMS, diisopyropylamine and triphenylphosphine were mixed together. Cuprous iodide and palladium acetate were added to the mixture along with tetrahydrofuran (THF). This mixture was stirred under argon over the course of a few days. This resulted in a formation of some precipitate. The reaction mixture was dissolved in benzene, and analyzed and found to comprise the reaction product bis-1,4[2-(4-(2-TMS)ethylnphenyl)ethynl] benzene (“the TMS compound”).

The TMS compound thus prepared was suspended in a methanol/ether solvent. Potassium hydroxide was added and this mixture stirred for two days. Thereafter, the solvent was removed by rotary evaporation and the resultant product slurried in ethyl ether and filtered through silica. Evaporation of the ether yielded a reaction product which was identified as bis-1,4[2-(4-ethylphenyl)ethynl] benzene (“the BIS 1,4 compound”).

The thus prepared BIS 1,4 compound was dissolved in methylene chloride. A platinum complex comprising bis-(tributylphosphine) platinum dichloride was dissolved in benzene. One drop of tributylphosphine was added and the mixture stirred for 10 minutes. Diisopyropylamine dissolved in methylene chloride was then added along with copper iodide into the solution of the BIS 1,4 compound. This mixture was stirred for approximately 60 hours. The solution thickened over that time, but no precipitate was formed. The solvent was evaporated to dryness, and the resultant solid dissolved in methylene chloride, precipitated and washed with methanol. The result of the synthesis was incorporation of platinum into the BIS 1,4 compound.

FIG. 1 is a chart that shows experimental data associated with the above procedure. FIG. 2 shows the absorption of Pt-3 (solution) along with excitation at 362 nm. FIG. 3 is a plot of a photoluminescent measurement of a polymer (Pt-3) solution according to the invention. FIG. 4 is a plot of photoluminescent measurement of Pt-3 polymer film, and FIG. 5 is a plot of photoluminescent measurement of Pt-3 polymer at different temperatures and phase variation.

Whereas, in the previous work, mixtures of multiple polymers were used to emit white or near-white light, according to this invention a few as a single π-conjugated polymer may be used with a heavy atom to realize organic light-emitting diodes (OLEDs) having a very high efficiency of conversion of electrical energy to plant growth. OLEDs whose emission spectrum match the absorption spectrum of plants may be constructed from two separate layers of organic semiconductors, including polymers and small molecules, again constituting an efficient light source for hydroponic applications. Indeed, these polymers have broad application for making illuminators (light sources) that show great promise of nearly perfectly matching the absorption spectrum of plants. Hence such illuminators would be an equally perfect source of light in hydroponic applications.

In keeping with the general procedure, it will be appreciated that the amount of platinum (or other heavy atom(S)) incorporated into the compound may be controlled by controlling the amount of platinum complex utilized in the reaction scheme. It will also be understood that one of skill in the art could readily substitute other organic compounds, inorganic compounds and organo compounds into the foregoing procedure so as to produce various materials.

As is known and apparent to those of skill in the art, organic light-emitting devices may be readily fabricated by coating the thus prepared materials onto the electrically conductive substrates such as glass coated indium tin oxide and the like. Materials and devices prepared in accordance with the invention secure the particular advantages of high output and tunable emissions so that a selected wavelength or band of wavelengths may be readily achieved through the use of relatively simple, heavy metal-containing single polymer having a tailored output.

The heavy atom acts to enhance phosphorescent emission which might otherwise not be seen at all. The invention allows for the chemical tuning of the emission bands through the placement of different spacers between adjacent Pt atoms in the polymers. The relative strengths of the phosphorescence to the luminescence may also be tuned by diluting the heavy atoms present in the chain; say, from one Pt atom on each monomer, to Pt atoms on every 3, 5, 7, etc.

At least some of our Pt-based polymer has an emission band in the blue-UV region of the spectrum that nearly perfectly matches one absorption band of plants. This polymer has a second band in the yellow-orange region of the spectrum. Based on the separation of these bands in other polymers, a polymer according to the invention may be engineered such that the lower energy emission is exactly at the needed wavelength for absorption by plants. We have experimented with other polymers that serve as active layers in various OLEDs, which emit in the red, again nearly perfectly matching the absorption of plants. As such, the needs of plants can be met with a single-layer OLED, which is technologically very easy and cheap to produce.

An OLED having two layers of polymer would still be a much more efficient fit to the spectral needs of plants than is the case for any existing artificial source, making a light source that would be orders of magnitude more efficient than an incandescent light and significantly more than fluorescent sources. In addition to the vast improvement in efficiency of conversion of electricity to useable light as needed by the “consumer,” the OLEDs can be produced in large quantities using inexpensive technologies like spin-casting. Equally inexpensive substrates like sheet metal or plastic sheet will significantly reduce the “capitalization cost” of the illuminators in the first place. Finally, since OLEDs typically work off sources in the neighborhood of 5 Volts, the device would avoid the use of 30,000-volt ballast transformers found fluorescent lamps, further improving the cost advantage. 

1. An organic light-emitting diode (OLED) particularly suited to plant growth and hydroponic applications, comprising: an active layer including a polymer having a π-conjugated backbone; and a heavy atom in the backbone causing the fluorescent to phosphorescent emission bands to overlap upon excitation, resulting in an emission tailored to the absorption spectra of growing plants.
 2. The OLED of claim 1, including plants grown under hydroponic conditions.
 3. The OLED of claim 1, wherein the emission occurs in both the blue-green and red regions of the visible spectrum.
 4. The OLED of claim 1, wherein the heavy atom is platinum.
 5. The OLED of claim 1, wherein the heavy atom is iridium.
 6. The OLED of claim 1, wherein the emission band is chemically tuned through the placement of different spacers between adjacent heavy atoms.
 7. A method of stimulating plant growth, comprising the steps of: providing a π-conjugated polymer; adding a heavy atom to the polymer causing its fluorescent to phosphorescent emission bands to overlap upon excitation; stimulating the mixture to emit light tailored to the absorption spectra of growing plants; and exposing plants to the light.
 8. The method of claim 7, wherein the heavy atom is platinum.
 9. The method of claim 7, wherein the heavy atom is iridium.
 10. The method of claim 7, including the step of placing different spacers between adjacent heavy atoms to tune the emission.
 11. The method of claim 7, wherein the plants are grown under hydroponic conditions.
 12. The method of claim 7, wherein the emission occurs in both the blue-green and red regions of the visible spectrum. 